elapsed time in C

Question

#include <time.h> 
time_t start,end; 
time (&start);
//code here
time (&end); 
double dif = difftime (end,start); 
printf ("Elasped time is %.2lf seconds.", dif );

I'm getting 0.000 for both start and end times. I'm not understanding the source of error.

Also is it better to use time(start) and time(end) or start=clock() and end=clock() for computing the elapsed time.

Can you post the code that you are timing? It looks like you are doing timing right, as long as you don't need high precision timing. — Thomas Owens, May 30 '11 at 18:06
This works fine for me. Have you verified the values of start and end with a debugger? (Note: to test it I put a "sleep(1)" between the time(start) and time(end).) — Tom, May 30 '11 at 18:07
If you need time measurements at finer resolution, have you tried `clock()`? — sverre, May 30 '11 at 18:08
@sverre: `clock()` does not measure the absolute passage of time. It measures the quantity of time spent specifically on this one process. Important distinction. If you're profiling, you may want this; otherwise you probably don't. — Lightness Races in Orbit, May 30 '11 at 19:06

score 7 · Answer 1 · answered May 30 '11 at 18:05

7

On most (practically all?) systems, time() only has a granularity of one second, so any sub-second lengths of time can't be measured with it. If you're on Unix, try using gettimeofday instead.

answered May 30 '11 at 18:05

jwodder

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score 4 · Answer 2 · answered May 30 '11 at 18:14

4

If you do want to use clock() make sure you understand that it measures CPU time only. Also, to convert to seconds, you need to divide by CLOCKS_PER_SEC.

answered May 30 '11 at 18:14

mhyfritz

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score 2 · Answer 3 · answered May 30 '11 at 18:17

Short excerpts of code typically don't take long enough to run for profiling purposes. A common technique is to repeat the call many many (millions) times and then divide the resultant time delta with the iteration count. Pseudo-code:

count = 10,000,000

start = readCurrentTime()

loop count times:
    myCode()

end = readCurrentTime()

elapsedTotal = end - start
elapsedForOneIteration = elapsedTotal / count

If you want accuracy, you can discount the loop overhead. For example:

loop count times:
    myCode()
    myCode()
and measure elapsed1 (2 x count iterations + loop overhead)

loop count times:
    myCode()
and measure elapsed2 (count iterations + loop overhead)

actualElapsed = elapsed1 - elapsed2
(count iterations -- because rest of terms cancel out)

score 0 · Answer 4 · edited May 23 '17 at 12:32

0

time has (at best) second resolution. If your code runs in much less than that, you aren't likely to see a difference.

Use a profiler (such a gprof on *nix, Instruments on OS X; for Windows, see "Profiling C code on Windows when using Eclipse") to time your code.

edited May 23 '17 at 12:32

Community

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answered May 30 '11 at 18:08

outis

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DallaRosa · Answer 5 · 2011-05-30T18:24:10.453

0

The code you're using between the measurements is running too fast. Just tried your code printing numbers from 0 till 99,999 and I got

Elasped time is 1.00 seconds.

edited May 30 '11 at 18:24

answered May 30 '11 at 18:12

DallaRosa

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score 0 · Answer 6 · answered May 30 '11 at 19:06

0

Your code is taking less than a second to run.

answered May 30 '11 at 19:06

Lightness Races in Orbit

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elapsed time in C

6 Answers6